The present invention generally relates to piezoelectric motors and similar devices. More specifically, the present invention relates to a unique electrical circuit to drive traveling wave piezoelectric motors and related piezoelectric devices.
Devices such as piezoelectric traveling wave motors, which will be referred to herein simply as piezoelectric motors, are devices which utilize deformation of small piezoelectric elements to provide motive force. The elements used in such devices are piezoelectric ceramics or crystals which deform when exposed to an electric field. By exposing a set of piezoelectric elements to a series of timed electrical pulses, a motion can be derived from the elements. Piezoelectric motors are increasingly found in common uses in today's society, such as in drives to operate lenses on automatic cameras, drives to operate curtains and shades, and a variety of other uses requiring small lightweight, inexpensive, and reliable motors.
Piezoelectric vibration devices, such as piezoelectric motors and actuators use an oscillating voltage electrical source to cause the piezoelectric elements to vibrate. In particular, traveling wave piezoelectric motors depend upon generation of two standing waves displaced both a quarter wavelength in time and in space to form a traveling wave in the motor stator. This is typically accomplished by feeding a sine wave and a cosine wave (i.e., a sine wave and a sine wave displaced a quarter wavelength in time) to elements displaced a quarter wavelength spatially around the stator. General motor circuit and timing considerations are discussed in the application of Charles Mentesana, U.S. application Ser. No. 08/628,141, filed Apr. 4, 1996 ("Mentesana") which is incorporated herein by reference. (Both Mentesana and the present application are assigned to a common entity.) In order to most effectively induce the traveling wave, piezoelectric motor drive circuits are designed to produce these standing and traveling waves at or near frequencies at which the motor is resonant.
One characteristic of piezoelectric motors is that the resonant frequency of the motor changes in accordance with the temperature and pressure experienced by various portions of the motor, and especially the piezoelectric elements. Changes in ambient temperature, heating of the motor elements due to friction, electrical resistance, and the like cause significant temperature shifts in the motor. Similarly, in operation, the motor rotor is in contact with the motor stator, the pressure of which contact varies in operation due to various factors including temperature changes and motor wear. As a result of these pressure and temperature changes, significant changes in motor operating frequency result during motor operation. It is noted that the resonant frequency is also dependent upon motor torque, motor configuration, and similar factors. However, the primary variable operational factors are pressure and temperature. For simplicity, references herein to temperature and pressure factors affecting the harmonic frequency, include such other factors.
One approach to driving these piezoelectric motors has been to generate electrical pulses with set frequencies to drive the motor. This approach limits the operating range and efficiency of the motor because the selected frequency generally is a compromise between the range of harmonic frequencies anticipated in motor operation.
Another approach is to utilize complex circuits to provide the required series of electrical pulses or waveforms in conjunction with complex schemes to modify the driving frequency to account for changes in the harmonic frequency of the piezoelectric elements due to temperature and pressure changes. Examples include U.S. Pat. No. 5,101,144 (Hirotomi); U.S. Pat. No. 5,130,598 (Verheyen), and U.S. Pat. No. 4,658,172 (Izukawa), which are hereby incorporated by reference. Generally, circuits similar to these are complex and costly to fabricate and can suffer from being sensitive to vibration and other circuit failures.
One approach has been to make use of the piezoelectric element as one leg of a full Meacham bridge circuit, such as generally is shown in U.S. Pat. No. 4,658,172. However, in using a full Meacham bridge approach, the power available to drive the piezoelectric element is severely limited because the current flow through the frequency-setting element is directly related to the input to the circuit amplifier. Basically, high current flows in the piezoelectric element overdrives the amplifier, thus limiting available current flow. Further, such circuits tend to be inefficient due to the number of passive circuit components required to form the bridge.
Other approaches utilize schemes to feed back pressure and temperature signals, and/or utilize those signals to modify the drive circuit frequency output. However, for example, to produce a temperature correction to the circuit, a thermocouple sensor may be utilized, which must be interconnected with the temperature correction circuitry. Added wiring such as this represents significant additional complexity in manufacturing as well as cost of the final installation. Further, because of their complexity, such circuits may be prone to component failure and may not provide an optimum operating frequency correction.